Bordetella bronchiseptica and Atrophic Rhinitis in Pigs: Turbinate Atrophy and Diagnosis

Atrophic rhinitis is a globally significant respiratory disease of swine characterized by progressive atrophy of the nasal turbinates leading to facial distortion, growth retardation, and reduced feed conversion efficiency. The primary bacterial agent initiating the cascade of turbinate destruction is Bordetella bronchiseptica, a Gram-negative coccobacillus that colonizes the upper respiratory tract. This article provides an exhaustive review of Bordetella bronchiseptica as the causative agent of atrophic rhinitis, with a specific focus on the mechanisms of turbinate atrophy, clinical presentation, pathological findings, and modern diagnostic strategies.

Etiology and Taxonomy

Bordetella bronchiseptica is a small, aerobic, motile, Gram-negative coccobacillus belonging to the family Alcaligenaceae. It is closely related to Bordetella pertussis and Bordetella parapertussis, the agents of whooping cough in humans, but B. bronchiseptica has a broad host range including swine, dogs, cats, rabbits, and laboratory rodents. In swine, it is the primary etiological agent of nonprogressive atrophic rhinitis (NPAR) and acts as a predisposing factor for progressive atrophic rhinitis (PAR) when coinfection with toxigenic Pasteurella multocida type D occurs. The organism expresses multiple virulence factors, including filamentous hemagglutinin, fimbriae, adenylate cyclase toxin, tracheal cytotoxin, and a dermonecrotic toxin (DNT) that is largely responsible for the osteolytic changes in the nasal turbinates.

Epidemiology and Transmission

B. bronchiseptica is endemic in most swine herds worldwide. Transmission occurs through direct contact, aerosol droplets, and fomites. Carrier sows are the primary reservoir and shed the bacterium to their offspring during the neonatal period. Piglets become infected within the first few days of life, and colonization peaks during the nursery phase. Herd-level prevalence can approach 100 percent in conventionally managed operations. The bacterium survives for limited periods in the environment but can persist in moist nasal secretions and water. Biosecurity lapses and introduction of replacement gilts from infected sources perpetuate endemicity.

Pathogenesis and Turbinate Atrophy

The process of turbinate atrophy is a direct consequence of B. bronchiseptica infection and its toxin-mediated effects on bone metabolism. Following inhalation, the organism adheres to ciliated respiratory epithelial cells using fimbriae and filamentous hemagglutinin. Tracheal cytotoxin, a peptidoglycan fragment, inhibits ciliary motility and causes ciliostasis, compromising mucociliary clearance. The bacterium then proliferates in the nasal cavity and releases dermonecrotic toxin (DNT), a heat-labile protein that enters osteoblasts and osteoclasts via receptor-mediated endocytosis.

DNT acts as a potent inhibitor of osteoblast differentiation and function. It disrupts the Rho GTPase signaling pathway, leading to cytoskeletal rearrangement, impaired mineralization, and increased apoptosis of bone-forming cells. Concurrently, DNT stimulates osteoclastogenesis, tipping the balance of bone remodeling toward resorption. The net effect is progressive loss of the ventral and dorsal nasal turbinate scrolls, which are composed of thin, trabecular bone. The turbinate bones become misshapen, shortened, and may be completely resorbed. This atrophy reduces the surface area of the nasal mucosa, compromising the warming and filtering functions of the nasal passages.

The term "Bordetella bronchiseptica atrophic rhinitis pigs turbinate atrophy" encapsulates the core pathophysiological triad: bacterial colonization, toxin release, and osteolytic remodeling. The severity of atrophy correlates with the dose of DNT and the age at infection. Younger piglets (under 4 weeks of age) are most susceptible because of their rapidly growing nasal bones and immature immune responses.

Clinical Signs

Clinical signs of atrophic rhinitis vary from subclinical to severe, depending on the infecting strain, herd immune status, and presence of co-pathogens. Sneezing is often the earliest sign, observable as paroxysms of snorting, especially after feeding or when pigs are disturbed. Serous to mucopurulent nasal discharge, epiphora (tear staining), and occasional epistaxis may be noted. As turbinate atrophy progresses, the bridge of the nose may become shortened, deviated, or wrinkled, leading to the characteristic "snout deformity." In severe cases, the upper jaw is shortened relative to the lower jaw, causing malocclusion and difficulty prehending feed.

Affected pigs exhibit suboptimal growth rates, reduced average daily gain, and poorer feed conversion ratios. These economic impacts are most pronounced in grower-finisher stages. However, many infected pigs remain subclinical with only mild sneezing and no overt snout distortion, yet still carry the bacterium and contribute to herd transmission. Concurrent infection with Pasteurella multocida type D exacerbates clinical severity, leading to progressive atrophic rhinitis (PAR) with pronounced turbinate destruction.

Pathology

Gross pathological changes are confined to the nasal cavity. At necropsy, the nasal passages are examined by making a transverse section at the level of the first premolar tooth (canine tooth in older pigs). The ventral and dorsal turbinate scrolls are evaluated for symmetry, size, and conformation.

Histologically, the turbinate bone shows thinning of trabeculae, decreased osteoblast numbers, and increased osteoclast activity on bone surfaces. The overlying respiratory epithelium may exhibit goblet cell hyperplasia, squamous metaplasia, and infiltration of neutrophils and mononuclear cells. In chronic cases, fibrosis of the submucosa is evident. The olfactory epithelium may be partially replaced by respiratory-type epithelium in advanced atrophy.

Diagnosis

Accurate diagnosis of B. bronchiseptica infection and atrophic rhinitis requires a combination of clinical, pathological, and laboratory methods. Given that subclinical infections are common, laboratory confirmation is essential for herd-level surveillance and control program decisions.

Clinical Examination and Necropsy

Sneezing, tear staining, and nasal discharge are nonspecific but raise suspicion. Snout palpation and visual inspection for deformities should be performed. At necropsy, turbinate scoring using the 0 to 4 system provides a semiquantitative assessment of atrophy. A herd mean score greater than 1.0 is considered indicative of active atrophic rhinitis.

Bacteriological Culture

Nasal swabs collected from live pigs (using deep swabbing of the nasal cavity) or turbinate tissue at necropsy can be cultured on selective media such as Bordet-Gengou agar or MacConkey agar. B. bronchiseptica appears as small, convex, glistening colonies after 48 hours at 37 degrees Celsius under aerobic conditions. The organism is oxidase-positive, urease-positive, and nonfermentative. However, culture sensitivity declines in chronically infected or antibiotic-treated pigs, and coinfection with other bacteria may overgrow the plates.

Molecular Diagnostics

Polymerase chain reaction (PCR) targeting the fla gene or the dermonecrotic toxin gene provides high sensitivity and specificity. Real-time PCR can quantify bacterial load. Nasal swabs, bronchoalveolar lavage fluid, or tissue homogenates are suitable samples. PCR can detect B. bronchiseptica even after antibiotic therapy and in cases with low bacterial numbers, making it the preferred method for confirmation.

Serology

Enzyme-linked immunosorbent assays (ELISAs) using whole-cell or outer membrane protein antigens can detect antibodies against B. bronchiseptica. Serological testing is useful for herd-level exposure assessment but does not differentiate active infection from past exposure or vaccination. Paired serology (acute and convalescent) is rarely used in swine practice because of the chronic nature of the disease.

Differential Diagnosis

Other causes of sneezing and nasal discharge in pigs include swine influenza virus, porcine reproductive and respiratory syndrome virus, Mycoplasma hyopneumoniae (though primarily lower respiratory), and noninfectious irritants such as high ammonia concentrations. Turbinate atrophy is pathognomonic for atrophic rhinitis, but the agent must be confirmed. Where coinfection with toxigenic Pasteurella multocida is suspected, PCR for the toxA gene should be performed.

Below is a diagnostic decision tree for a suspected case of atrophic rhinitis.

flowchart TD
    A[Clinical suspicion: sneezing, tear staining, snout deformity], > B{Nasal swab for PCR}
    B, > C[Positive for B. bronchiseptica]
    B, > D[Negative for B. bronchiseptica]
    C, > E[Assess severity: turbinate scoring at necropsy or CT imaging]
    D, > F[Rule out other pathogens: swine influenza, PRRSV, pasteurellosis]
    E, > G[Score >= 2: confirm atrophic rhinitis]
    G, > H[Test for P. multocida toxA if progressive lesions]
    H, > I[Implement control measures: vaccination, biosecurity, management]

Treatment and Antimicrobial Considerations

Antimicrobial therapy can reduce clinical signs and bacterial shedding but rarely eliminates B. bronchiseptica from a herd because of the intracellular niche and biofilm formation. Susceptibility testing is recommended given the emergence of resistance. Effective agents include tetracyclines, sulfonamides, ampicillin, and ceftiofur, administered by injection or in-feed medication. However, widespread prophylactic use in weaner pigs has contributed to resistance. In many regions, isolates show decreased susceptibility to tetracycline and sulfamethazine. Treatment should be targeted to affected age groups and combined with management improvements.

Control and Prevention

Control of atrophic rhinitis caused by B. bronchiseptica relies on integrated strategies.

1. Vaccination: Commercial bacterins and toxoid vaccines containing B. bronchiseptica whole cells and inactivated DNT are available for sows and piglets. Vaccination of sows pre-farrowing boosts maternal antibody transfer to piglets via colostrum, reducing early colonization. Piglet vaccination at 1 and 3 weeks of age provides some protection. Vaccines are most effective when combined with management measures.

2. Biosecurity: All-in/all-out flow, cleaning and disinfection between groups, and strict rodent and bird control reduce pathogen introduction and persistence. Replacement stock should be sourced from herds with low turbinate scores and negative PCR status.

3. Medication: Strategic in-feed antibiotics during high-risk periods (e.g., postweaning) can reduce bacterial load but should be used judiciously to avoid resistance.

4. Eradication: Herd depopulation and repopulation with B. bronchiseptica free stock is feasible but costly. Partial depopulation with early weaning and medication has had limited success.

5. Environmental management: Reducing ammonia levels, dust, and overcrowding minimizes respiratory irritation and mucosal damage that facilitate colonization.

Conclusion

Bordetella bronchiseptica remains a primary bacterial cause of atrophic rhinitis in pigs, driving turbinate atrophy through the action of dermonecrotic toxin on osteoblasts and osteoclasts. The condition leads to significant economic losses due to reduced growth and feed efficiency. Accurate diagnosis requires integration of clinical observation, necropsy turbinate scoring, and laboratory detection by PCR or culture. Control relies on vaccination, biosecurity, and prudent antimicrobial use. Understanding the interplay between B. bronchiseptica and co-pathogens such as Pasteurella multocida is essential for managing the progressive form of the disease. Ongoing surveillance for antimicrobial resistance and refinement of molecular diagnostic tools will further improve herd health outcomes.

References

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2. Straw BE, Zimmerman JJ, D'Allaire S, Taylor DJ. Diseases of Swine. 9th ed. Blackwell Publishing.
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4. Magyar T, Lax AJ. Atrophic rhinitis. In: Brogden KA, Guthmiller JM, editors. Polymicrobial Diseases. ASM Press.